The present invention relates broadly to an RF modulator apparatus, and in particular to double sideband generation with serrodyne modulators.
The state of the art of RF signal modulators is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. patents:
U.S. Pat. No. 2,462,852 issued to Frankel on Mar. 1, 1949; PA1 U.S. Pat. No. 2,566,876 issued to Dome on Sept. 4, 1951; PA1 U.S. Pat. No. 3,320,552 issued to Glasser on May 16, 1967; and, PA1 U.S. Pat. No. 3,562,671 issued to Takamicki Honma et al on Feb. 9, 1971.
U.S. Pat. No. 2,462,852 discloses a frequency modulation system for producing voltage undulations of a generally sawtooth pattern. U.S. Pat. No. 2,566,876 discloses a phase shift system for obtaining from a single source of voltage two resultant voltages which are constant in amplitude, but which differ in phase by a substantially constant phase angle. U.S. Pat. No. 3,562,671 discloses a pulse position modulation system which includes means for suppressing zero-modulation signal components. U.S. Pat. No. 3,320,552 discloses a band limited frequency modulation system for generating a bandwidth limited frequency modulated signal.
In FIG. 1 there is shown a graphical representation of the output spectrum of a double sideband, suppressed-carrier modulator (DSBSC), wherein the RF carrier is at the frequency, f(RF) and the lower and upper sidebands are at the frequencies, f(LSB) and f(USB), respectively. The upper and lower sideband frequencies are offset from the carrier frequency, f(RF) by the modulation frequency, f(MOD). In addition, there is shown the spurious products that are generated during the modulation process. It will be noted that in the prior art, various circuit configurations of the type of modulator that will produce this typical frequency spectrum can be readily found. There is also readily available a number of prior art circuit configurations that permit double sideband generation with the carrier and unwanted sideband levels of lower than -10 dBc dBc (referenced to the shifted carrier), where the actual suppression values are primarily dependent upon carrier bandwidth and operating temperature range.
If suppression values below -25 dBc are required, the conventional prior art double sideband modulation techniques, such as the double-balanced ring modulators or balanced attenuators, cannot be effectively utilized. Furthermore, the commonly known sophisticated schemes that use combinations of the conventional modulation approaches to meet the higher values of suppression are complex and expensive as well as completely unproven when using the presently available production technology.
It is possible to obtain carrier and sideband suppression below -25 dBc by using the digital serrodyne approach which is shown in FIG. 2. In this prior art approach, individual phase bits are switched into the RF path in order to produce the upconversion or downconversion serrodyne patterns that are respectively shown in FIGS. 3a and 3b. It is well-known in the present state of the art, that the carrier may be shifted by the serrodyne modulation rate and that the unwanted sideband level is primarily dependent upon the resolution of the phase steps. For example: 3-bit (i.e. 45 degree) phase steps will suppress unwanted sidebands by more than 16 dB whereas 6-bit resolution is necessary to achieve sideband suppression greater than 32 dB. Unfortunately, the modulation is only single sideband and there is no simple way of producing double sideband generation with a serrodyne modulator. The present invention provides a unique circuit combination of serrodyne frequency modulators to address the problem of generating double sideband suppressed carrier modulation when using serrodyne modulators.